Obesity runs in families - spouses enter marriage with similar weight statuses, gain weight together over time and model obesogenic behaviors for their children. Many energy balance decisions are made in the home and this important social/interpersonal environment can either help or hinder weight management. Ecological models of obesity recognize the impact of the home and larger environment contexts on weight, however, existing treatment models remain individual-focused. Weight loss outcomes might be improved by expanding the focus of treatment from the individual to the marital dyad, targeting joint behavior change and the creation of an interpersonal milieu that supports long-term behavior change. According to Self Determination Theory, greater autonomous self-regulation of behaviors and subsequently better treatment outcomes are observed when individuals have important others in their lives who provide an environment in which personally meaningful choice is supported and criticism and control are minimized. Previous research including our own ongoing work suggests that increases in autonomy support from important others are associated with increases in autonomous self-regulation for weight loss and that these changes in autonomous self-regulation are indeed associated with weight loss outcomes. Additional research is needed to establish causal links and to develop and test interventions designed to facilitate the autonomy supportive behavior of significant others in the home environment. This theoretically based approach has tremendous potential to add to the modest effects found in the published literature on the involvement of spouses in weight loss treatment. The proposed study is a randomized controlled trial testing a couple's intervention based on Self Determination Theory that is designed to change the social context of weight loss by training spouses to provide autonomy support for each other's eating and physical activity behavior change. Sixty-four couples will be randomized to either 6 months of treatment based on SDT (SDT-WL) or to 6 months of treatment based on behavioral theory (BWL). Couples will attend weekly sessions for 6 months and will be assessed at 0, 6, and 12 months. We predict that by bolstering autonomy support, SDT-WL will increase autonomous self-regulation and perceived competence and will produce greater weight loss and maintenance than standard behavioral treatment. The study focuses on couples rather than individuals, is theoretically based, focusing on autonomy support and distinguishing this from other types of support, and directly trains partners in such supportive behaviors. If successful, we will seek funding to test the SDT approach in a larger trial with a longer follow-up period.